Bipolar junction transistors are active semiconductor devices formed by a pair of P-N junctions, including an emitter-base junction and a collector-base junction. An NPN bipolar junction transistor has a thin region of p-type material providing the base region between two regions of n-type material providing the emitter and collector regions. A PNP bipolar junction transistor has a thin region of n-type material providing the base region between two regions of p-type material constituting the emitter and collector regions. The movement of electrical charge carriers which produces electrical current flow between the collector region and the emitter region is controlled by an applied voltage across the emitter-base junction.
Conventional vertical NPN bipolar devices used for electrostatic discharge (ESD) protection of other devices typically include an n+ buried layer (NBL) together with a n+ sinker diffusion as collectors in each device stripe (or finger) to provide a low resistance-path to carry ESD strike induced current back to the top surface of the substrate (e.g., a top silicon surface). In BiCMOS technologies, it is a common practice to use vertical NPN transistors with an NBL as the collector and a deep n+ sinker diffusion to form the current return path for the ESD protection circuitry.